This invention relates to an encoding system for use in carrying out encoding of a sequence of digital signals into a sequence of encoded signals with a high efficiency. It is to be noted here that the digital signal sequence may convey an audio signal, although description will be made about an image signal, and that such encoding will be called efficient coding in the instant specification.
In order to carry out efficient coding of the type described, a sequence of digital signals is subdivided in each frame into a plurality of blocks and is subjected to a predetermined linear transform in the respective blocks. Such a linear transform may be, for example, Hadamard transform, Haar transform, Karhunen-Loeve transform, Discrete Cosine transform, or the like, and may collectively be called an orthogonal transform. At any rate, the digital signals of each block are converted into transform coefficients which have a statistic distribution in a frequency region between a d.c. component zone and a high frequency zone and which have variable levels of electric power.
It is known in the art that electric power of the transform coefficients is locally distributed or concentrated on a local frequency zone which includes the d.c. component and a low frequency near the d.c. component. This shows that non-zero or significant ones of the transform coefficients mainly appear in the low frequency zone and scarcely appear in the high frequency zone while zero or nonsignificant ones of the transform coefficients mainly appear in the high frequency zone. Such nonsignificant coefficients need not always be transmitted through a transmission line.
Under the circumstances, the transform coefficients are preliminarily processed prior to quantization of the transform coefficients, which will be called anterior processing. A conventional encoding system monitors as the anterior processing a distribution of the significant coefficients in each of the blocks to determine a minimum zone which covers all of the significant coefficients and to transmit only the coefficients of the minimum zone, together with a code assigned to the minimum zone in question. This encoding system may be called a zone detection system. With this zone detection system, transmission is not carried out with respect to nonsignificant coefficients which are outside of the minimum zone.
Alternatively, another conventional encoding system detects a final significant coefficient in each block and produces an End Of Block (EOB) code which is representative of the continuation of nonsignificant coefficients after the EOB code. Such detection and production of the EOB code are carried out as the anterior processing. In order to effectively detect the EOB code, zigzag scanning is carried out in each block. The encoding system may be referred to as an EOB code system and is effective to reduce redundancy of the coefficients.
Furthermore, still another conventional encoding system carries out the anterior processing by adaptively scanning the transform coefficients of each block in various manners to decide an optimum one of scanning operations that gives a maximum length of continuation of nonsignificant coefficients after a final significant transform coefficient. This system may be called a scanning encoder system and is helpful to reduce an amount of information to be transmitted.
Also, it is to be noted here that significant transform coefficients have a tendency to be isolated from the other significant transform coefficients in the high frequency zone, although such isolated significant transform coefficients seldom appear in the high frequency zone, as mentioned before.
According to the inventors' experimental studies, it has been found that the removal or neglect of the isolated significant transform coefficients enables a considerable reduction in amount of information but brings about a reduction of a picture quality from time to time. Accordingly, the experimental studies say that complete neglect of the isolated significant coefficients in the high frequency zone is not favorable in the light of a reduction of the picture quality.
At any rate, the transform coefficients are quantized by a quantizer into a sequence of quantized signals after the above-mentioned anterior processing and are thereafter usually subjected by an encoding unit to variable length encoding which may be called posterior processing. Such posterior processing is carried out by the use of a single conversion table between the quantized signals and variable length codes. This means that the transform coefficients are encoded by the same conversion table, irrespective of the fact that they are present in the low and the high frequency zones. However, it has also been confirmed that the use of the same conversion table is not favorable in view of accomplishing a high efficiency because of the local concentration of the transform coefficients as mentioned before.